1. Field of the Invention
The present invention relates to a method of manufacturing a highly dense multi-layer printed wiring board using laser drilling.
2. Description of the Related Art
FIG. 5 is a diagram illustrating a conventional method of manufacturing a multi-layer printed wiring board disclosed in Japanese Patent Laid-Open No. Hei 9-246728. According to the conventional method of manufacturing a multi-layer printed wiring board, a varnish is applied to an organic film or a metal foil, and the solvent is removed by drying to form an insulating adhesive layer. Then, a cover film is stuck thereon, and the surface of the film is irradiated with a laser beam to form a non-through hole. Next, the non-through hole is filled with conductive paste, which is then half-cured by heating. Thereafter, the cover film is peeled off to form a single layer substrate. Then, the single layer substrate of the single layer is overlapped in position on the surface of the inner substrate having an inner circuit formed therein in such a manner that the metal foil is on the outer side, with the application of pressure and heat. Finally, the metal foil is etched to produce a multi-layer printed wiring board having a conductive pattern formed thereon.
In the conventional method of manufacturing multi-layer printed wiring board, the interlayer insulating layer (insulating adhesive layer) has not been reinforced with a fiber or the like and has a low rigidity. Therefore, if there exists a difference in the coefficient of thermal expansion between the inner board and the interlayer insulating layer, the wiring board warps when it is heated for laminating the interlayer insulating layer, resulting in a decrease in the reliability of junction portions of the mounting parts.
This invention was accomplished in order to eliminate such a problem, and provides a method of manufacturing highly reliable and highly densely printed wiring board which are not warped even by the application of pressure and heat, by using an uncured resin sheet reinforced with a fiber as an interlayer insulating layer, forming non-through holes by using a laser beam, filling the non-through holes with a conductive resin, temporarily curing the conductive resin and, then, connecting it to the wiring substrate by the application of pressure and heat.
The method of manufacturing multi-layer printed wiring board of this invention comprises the steps of:
(a) stacking to adhere a conductive film on at least one surface of an uncured adhesive resin sheet containing a fiber or a filler;
(b) irradiating a predetermined portion on the other surface of said uncured resin sheet with a laser beam to form, in said uncured resin sheet, a through hole which is tapered in cross section to expose said conductive film;
(c) filling said through hole with a conductive material to form an interlayer conductor to render the one surface and the other surface of said uncured resin sheet conductive to each other;
(d) stacking to adhere the other surface of said uncured resin sheet containing the portion of forming said interlayer conductor on at least one surface of an inner substrate that has wiring layers on both surfaces thereof;
(e) curing said uncured resin sheet; and
(f) patterning said conductive film into a predetermined wiring shape.
According to the method of manufacturing multi-layer printed wiring board of the invention, a non-adhesive resin sheet may be overlapped on the other surface of the uncured resin sheet in the step (a), and the non-adhesive resin sheet may be peeled off between the step (c) and the step (d).
According to the method of manufacturing multi-layer printed wiring board of the invention, the conductive material may be an uncured conductive material, the uncured conductive material may be half-cured to form a half-cured conductive material in the step (c), and the half-cured conductive material may be cured in the step (e).
According to the method of manufacturing multi-layer printed wiring board of the invention, the half-cured resin sheet may contain any fiber of glass, wholly aromatic polyamide or polyacrylate.